In a wireless communication system, a radio access network (RAN) radiates to define one or more wireless coverage areas. Each wireless coverage area may serve a plurality of wireless communication devices (WCDs) via an air interface. In densely-populated areas, wireless service providers may improve wireless coverage by overlaying several wireless coverage areas on substantially the same physical locations. Each wireless coverage area preferably uses non-overlapping or orthogonal carrier frequencies or carrier frequency bands so that the wireless coverage areas do not interfere with one another.
For example, a wireless service provider may use six or seven non-overlapping carrier frequencies to provide coverage in a city center, while using only two or three such bands in the city's suburbs. But even in densely-populated cities, there may be pockets where fewer carrier frequencies are required. Similarly, in less densely-populated locations, certain locations such as shopping malls may attract large numbers of wireless users. Accordingly, wireless service providers may overlay additional carrier frequencies in these locations in order to serve the needs of their customers.
While overlaying multiple frequencies on a physical location generally improves the wireless service in that location, it can also introduce problems. When a WCD associated with a wireless coverage area using a first carrier frequency moves from a first location with a greater number of wireless coverage areas to a second location with a smaller number of wireless coverage areas, the second location's wireless coverage areas may not support the first carrier frequency. Consequently, the WCD may have to tune to a second carrier frequency that is supported by a wireless coverage area in the second location. While the WCD is tuning, the WCD may miss certain messages sent to it by the RAN. This can result in the WCD's user experiencing poor service.
In one such case, an incoming communication session for the WCD arrives at the RAN, and the RAN needs to notify the WCD of the session. This session may be, for example, a voice call, a multimedia call, or data communication, and system behavior may differ based on whether the WCD is “idle” or “active.” An “idle” WCD is not already on another voice call or otherwise involved in an ongoing communication session. An “active” WCD is already on an ongoing voice call or otherwise involved in an ongoing communication session.
For purposes of example, consider the case of an incoming voice call. To notify the WCD of the incoming voice call, the RAN may page the WCD. An idle WCD that receives a page will typically indicate to the end user that they are being called (i.e., the WCD will “ring”). On the other hand, an active WCD that receives a page will typically indicate to the end user that a new call is incoming and may give the user an option to switch from the ongoing call to the new call (i.e., “call waiting”).
Typically, a RAN pages a WCD by transmitting page request messages on a paging channel. The paging channel is an air interface channel used for to paging functions, and is associated with the carrier frequency of its wireless coverage area. The RAN may support one or more paging channels per wireless coverage area. Each paging channel may be divided, via time division multiplexing, into a number of time slots, in some of which the RAN can transmit a page request message to a particular WCD.
A WCD preferably conserves battery power by listening to the paging channel only during specific time slots. A given “slot cycle index” (SCI) defines the time slots at which the WCD will check the paging channel for a page request message. For instance, under cdma2000®, a WCD operating at slot cycle index 0 (zero) would listen for a page request message every 1.28 seconds, a WCD operating at slot cycle index 1 would listen for a page request message every 2.56 seconds, a WCD operating at slot cycle index 2 would listen for a page request message every 5.12 seconds, and so on. The SCI of a particular WCD is typically configured in the RAN, but may be overridden by the WCD, or may be negotiated between the WCD and RAN. Furthermore, the SCI may take on values less than 0 (zero) and greater than 2.
When the WCD receives a page request message, it will normally respond to the RAN with a page response message. Under cdma2000®, a WCD preferably transmits the page response message to the RAN on an access channel, which is another type of channel supported by the air interface. After the RAN receives the page response message from the WCD, the RAN preferably affirms the receipt of the page response message by transmitting a base station acknowledgement message to the WCD on the paging channel.
For an idle WCD, once this procedure has completed, the RAN may allocate one or more traffic channels for the WCD to use for voice and/or data communication. For an active WCD, if the user indicates that he or she wants to switch to the new incoming call, the RAN may either reuse the existing traffic channels assigned to the WCD for the ongoing call, or the RAN may tear down these traffic channels and allocate new traffic channels for the new incoming call.
From time to time, messages transmitted between the RAN and WCD will fail to reach their destination. When such a failure happens, the RAN may try re-paging the WCD by transmitting another page request after waiting a period of time defined by the RAN's “paging interval.” A paging interval is defined as time period that the RAN will wait before attempting to re-page a WCD. The paging interval may be independent from and not synchronized with the time slots defined by the WCD's SCI. In some networks, for example, the paging interval may be set to 7-10 seconds. This means that when a page request message is not received by a WCD, the RAN may wait one or more paging channel time slots, where these slots are defined by the WCD's SCI, before attempting to re-page the WCD.
From a user-experience perspective, it is advantageous for the WCD to receive page requests as quickly as possible. While the WCD may be successfully paged on the second or third attempt, every paging interval that the RAN waits adds delay to the establishment of the communication session. For example, consider the case of an incoming voice call where the first two page request messages are missed. In a RAN with a paging interval set to 7 seconds, a WCD could experience an additional 14 seconds or more of delay during call setup. Call setup latency is a key performance metric of wireless communication systems. Callers who experience long delays between placing a call and being notified that the call has been successfully cut-through (e.g., the caller hears a ringback tone), may think that the call has failed even though it has not. This may result in callers unnecessarily re-trying calls, which puts additional load on the service provider's network, and frustrates the callers.
Furthermore, the RAN will typically attempt to page the WCD a fixed number of times, usually a series of 2 to 5 attempts, before giving up. When the RAN gives up paging a WCD, the incoming communication session fails to be established, and can also result in a poor user experience and frustrated callers. Thus, the service provider needs to balance the pros and cons of various paging strategies, in accordance with a WCD's SCI.